1. Field of the Invention
This invention relates to a steel column base plate member, and more particularly to a steel base member for connecting a steel column member of a steel structure to concrete foundation therefor.
2. Description of the Prior Art
Steel column members of architectural buildings or construction structures are connected to concrete foundations, by means of base plates. It is well known that the steel column is stronger than the concrete of the foundation by a factor of not smaller than 10. To compensate for such difference of the strength between the concrete of the foundation and the steel column, the lower end of the column is joined to a steel plate, and the base plate is secured to the concrete foundation by means of anchor bolts embedded in the concrete foundation. The joints between the column and the base plate and between the base plate and the anchor bolts may be effected by riveting or by welding or by bolt-and-nuts. To supplement the rigidity of the base plate, suitable ribs are integrally secured to the base plate. Whereby, mechanical load or stress acting on the steel column is transmitted to the concrete foundation through the entire contact area between the base plate and the concrete foundation, so as to avoid any excessive stress concentration in the concrete foundation.
More particularly, it has been a practice to spread the load of the steel column to the surface area of the base plate, most frequently ribbed base plate, for the purpose of preventing the concrete foundation from breakdown by direct application of the high steel column stress to the concrete foundation. The tensile stress from the steel column is borne by the anchor bolts. The dimensions of the base plate, the anchor bolts and the concrete foundation are designed on the basis of the aforesaid stress transmission from the steel column to the concrete foundation.
Generally speaking, the mechanical loading to the steel column of building and construction structure includes axial compression, bending moment, and shearing stress. The aforesaid three elements of the mechanical loading are simultaneously applied to the steel column, and the concrete portion of the foundation and the anchor bolts jointly bear such mechanical loading. The concrete portion of the foundation produces a reaction force to be applied to the base plate, in response to that portion of the mechanical loading which is borne by the concrete. When the axial tension is high in the steel column, the anchor bolts will bear such high axial tension.
Accordingly, the base plate is required to fulfill the following conditions.
1. Since a large grounding force is applied to the base plate, the base plate must have a sufficiently large mechanical strength and rigidity to withstanding against outward bending moment (positive bending moment). PA1 2. The anchor bolts are sometimes subjected to tension. In this case, a reactive force is generated in the proximity of those bolt holes of the base plate which are for the anchor bolts subjected to the tension. Such reaction tends to cause an outward bending stress (negative bending moment), so that the base plate should also have a sufficient strength and rigidity for withstanding against such outward bending stress. PA1 3. The bending moment and shearing force which act on the column base plate are caused by earthquakes and typhoons, so that such moment and force are alternatingly oriented to different directions. Thus, the strength and the rigidity of the base plate should be symmetrical with respect to the vertical central axis of the steel column. The base plate is required not only to withstand against any foreseen load (breakdown strength), but also to restrict the magnitude of strain or deformation thereof (rigidity). PA1 a. The steel column is directly joined to the base plate by welding, and the welding tends to cause strain in the base plate. Due to such welding strain, sometimes, it has been difficult to achieve stable planar contact surface between the steel base plate and the concrete foundation. To ensure the planar contact, the welding strain is removed by extra treatment, such as heating or grinding, but such extra treatment means an additional working time and cost. PA1 b. The welding of the reinforcing rib plates to the base plate tends to further increase the welding stress in the base plate. Despite such risk, rib plates are actually welded to the base plate, because the ribbed base plate gives the best economy. PA1 c. To obtain maximum improvement of the rigidity of the base plate, it is desirable to weld the rib plates at small intervals. The small rib intervals are, however, detrimental to sound welding thereof, and it becomes difficult to tighten the anchor bolts when the rib intervals are small. PA1 d. The reinforcing ribs present a complicated surface shape to the base plate. Dust particles and moisture are apt to be trapped in the complicatedly shaped surface, which may accelerate the corrosion of the base plate to shorten the service life thereof.
To meet such requirements of dynamic and static nature, those portions of the base plate which are exposed to the effect of predicted high bending stresses should have a sufficient thickness for ensuring the strength necessary for bearing such effect of the stresses. On the other hand, it has been a practice to use a base plate of uniform thickness. Accordingly, the thickness of the conventional base plate must be selected, on the basis of the required thickness at that portion of the base plate where the maximum outward bending stress is caused. In practice, a comparatively thin steel plate is used for the base plate on the basis of normal maximum stress, for the purpose of economy. To supplement the strength, rib plates are secured to the base plate. The joint of the rib plates to the base plate cooperates with the joint of the steel column to the base plate in dividing the base plate in sections, so as to bear the outward bending moment acting thereto by the sections thus formed. Such division results in a reduction of the magnitude of the outward bending moment, and it also increases the resistance of the base plate against deformation.
The conventional base plate of the aforesaid structure has the following shortcomings.
Therefore, an object of the present invention is to mitigate the aforesaid difficulties of the conventional base plates by providing an improved steel column base plate member.